Regarding a steering device of a vehicle, for example, JP 2009-298298 A (Reference 1) discloses a “steering column” (see paragraphs [0024] to [0029] and FIGS. 1 and 2 of Reference 1), and discloses a “telescopic actuating mechanism” (see paragraphs [0030] to [0032] and FIGS. 6 and 7 of Reference 1). In addition, JP 2015-083396 A (Reference 2) discloses a “steering device that absorbs energy applied to a steering shaft of a vehicle,” that is, a steering device of a vehicle which includes an inner tube that accommodates the steering shaft of the vehicle so as to prevent a relative movement of the steering shaft in the axial direction, and to support the steering shaft to be rotatable about an axis and an outer tube that accommodates the inner tube so as to hold the inner tube at a predetermined position in a normal state, and the steering device is configured to allow a relative movement of the inner tube with respect to the outer tube in the axial direction when a load equal to or larger than a predetermined value is applied to the steering shaft (see paragraphs [0001] and [0007] of Reference 2).
The “first tube,” “second tube” and “third tube” in the steering column described in Reference 1 respectively correspond to “column housing,” “outer tube” and “inner tube” described in Reference 2, and in each steering column, a column tube is constituted with an inner tube that accommodates a steering shaft so as to support the steering shaft to be rotatable about the axis and the outer tube that accommodates the inner tube so as to hold the inner tube at a predetermined position in a normal state. Particularly, as a premise of a unit of absorbing energy applied to the steering shaft, since the steering shaft is supported with respect to the inner tube such that the relative movement of the steering shaft in the axial direction is prevented, the inner tube is made of a high specific gravity material such as iron in order to ensure the required rigidity. In a device including a telescopic actuating mechanism as described in Reference 1, since a “nut frame” constituting the actuating mechanism is fixed to the outer tube (the second tube), the outer tube is also made of a high specific gravity material such as weldable iron. Consequently, both the inner tube and the outer tube constituting the column tube have to be made of a high specific gravity material, and thus not only it is very hard to reduce the weight but also it is difficult to save the space.
Thus, a need exists for a steering device of a vehicle which is not susceptible to the drawback mentioned above.